Dye-sensitized solar cell and method for fabricating the same

ABSTRACT

A dye-sensitized solar cell and method for fabricating the same are provided. The dye-sensitized solar cell includes a photo electrode including a first electrode and a Zn-doped TiO 2  porous layer disposed on the first electrode, wherein the Zn-doped TiO 2  porous layer absorbs a dye. A second electrode is disposed opposite to the photo electrode, wherein the Zn-doped TiO 2  porous layer is disposed between the first and second electrodes. An electrolyte is disposed between the photo electrode and the second electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of Taiwan Patent Application No. 098141672, filed on Dec. 7, 2009, the entirety of which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a dye-sensitized solar cell and a method for fabricating the same, and in particular, to a photo electrode of a dye-sensitized solar cell and a method for fabricating the same.

2. Description of the Related Art

Solar energy is a promising alternative energy source. Developments in solar energy technology are driven by the fact that solar energy is continuous and sustainable. Also, solar energy is environmentally friendly.

Generation of solar energy requires solar cells. Simplistically, electricity is generated by circuitry, generated by illumination of a material to generate electrons and holes. For example, a dye-sensitized solar cell (DSSC) is formed by sintering a semiconductor type metal oxide such as TiO₂ on a conductive substrate. A dye is then absorbed by a surface of the semiconductor type metal oxide to form a photo-anode. An electrolyte between the photo-anode and a cathode aids in electrical conduction. Currently, technological development of DSSCs is popular due to its high photo-induced electron transfer efficiency and low costs. However, for a DSSC applied in a portable power supply, electron loss resulting in decreased photo-induced electron transfer efficiency is a problem when the DSSC is under a low light intensity environment.

Meanwhile, conventionally used semiconductor photocatalytic materials, may comprise TiO₂, ZnO, SnO₂ or CdS. Of the materials, TiO₂ is a popular material due to its high oxidation-reduction ability, high chemical stability and the fact that it is non-poisonous. The conventional metal ion-doped photocatalytic materials are mainly applied to improve photocatalytic activity by trapping oxides to increase the absorption of light.

Thus, a novel dye-sensitized solar cell and a method for fabricating the same are desired with minimum electron loss under a low light intensity environment, resulting in increased photo-induced electron transfer efficiency.

BRIEF SUMMARY OF INVENTION

A dye-sensitized solar cell and a method for fabricating the same are provided. An exemplary embodiment of a dye-sensitized solar cell comprises a photo electrode comprising a first electrode and a Zn-doped TiO₂ porous layer disposed on the first electrode, wherein the Zn-doped TiO₂ porous layer absorbs a dye. A second electrode is disposed opposite to the photo electrode, wherein the Zn-doped TiO₂ porous layer is disposed between the first and second electrodes. An electrolyte is disposed between the photo electrode and the second electrode.

An exemplary embodiment of a method for fabricating a dye-sensitized solar cell comprises: (a) mixing a TiO₂ powder, a Zn-containing compound and an alkaline aqueous solution to form a mixture and performing a thermal process on the mixture to form a Zn-doped TiO₂ powder; (b) mixing a binder solution with the Zn-doped TiO₂ powder to form a paste; (c) coating the paste on a first electrode, and the paste is sintered to form a Zn-doped TiO₂ porous layer, wherein the Zn-doped TiO₂ porous layer and the first electrode construct a photo electrode; (d) disposing a second electrode opposite to the photo electrode after a dye is absorbed by the Zn-doped TiO₂ porous layer; and (e) disposing an electrolyte between the photo electrode and the second electrode.

A detailed description is given in the following embodiments with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram showing one exemplary embodiment of a dye-sensitized solar cell;

FIG. 2 is an x-ray diffraction diagram showing various exemplary embodiments of a photo electrode formed by a Zn-doped TiO₂ porous layer of the invention;

FIG. 3 is a Mott-Schottky plot showing one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer of the invention;

FIGS. 4 a to 4 b are energy band diagrams showing the conventional photo electrode formed by undoped TiO₂ and the one exemplary embodiment of photo electrode formed by a Zn-doped TiO₂ porous layer of the invention;

FIG. 5 is a current density-voltage (J-V) curve comparison between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of photo electrode formed by a Zn-doped TiO₂ porous layer of the invention;

FIG. 6 a is a short-circuit current density/light intensity-light intensity curve comparison between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of photo electrode formed by a Zn-doped TiO₂ porous layer of the invention; and

FIG. 6 b is a photoelectric conversion efficiency-light intensity curve comparison between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of photo electrode formed by a Zn-doped TiO₂ porous layer of the invention.

Table. 1 shows flat band voltage and donor density comparisons between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of photo electrode formed by a Zn-doped TiO₂ porous layer of the invention.

DETAILED DESCRIPTION OF INVENTION

The following description is of a mode for carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. Wherever possible, the same reference numbers are used in the drawings and the descriptions to refer the same or like parts.

The present invention will be described with respect to particular embodiments and with reference to certain drawings, but the invention is not limited thereto and is only limited by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual dimensions to practice of the invention.

One exemplary embodiment of the invention provides a dye-sensitized solar cell, wherein a photo electrode of the dye-sensitized solar cell comprises a Zn-doped TiO₂ porous layer having a continuous electron transport path, thereby minimizing electron loss and increasing photoelectric conversion efficiency; especially in low light intensity environments.

FIG. 1 is a schematic diagram showing one exemplary embodiment of a dye-sensitized solar cell (DSSC) 500. The DSSC 500 may comprise a photo electrode 201 and an opposing second electrode 202, wherein the photo electrode 201 comprises a first electrode 200 and a Zn-doped TiO₂ porous layer 204 disposed on the first electrode 200. In one embodiment, the first electrode 200 may comprise a transparent electrode, for example, a fluorine tin oxide (FTO) glass or an indium tin oxide (ITO) conductive glass, to enable outside light to transmit through the first electrode 200 into the DSSC 500. The second electrode 202 may serve as a counter electrode used to transmit the electrons from the peripheral circuit into the DSSC, thereby forming a current circuit. The second electrode 202 may comprise Pt, conductive polymer, carbon or composite materials. A Zn-doped TiO₂ porous layer 204 absorbing a dye 208 is disposed between the first and second electrodes 200 and 202. In one embodiment, the dye 208 may comprise organic dyes or inorganic dyes. An electrolyte 206 is disposed between the photo electrode 201 and the second electrode 202. In one embodiment, the electrolyte 206 may comprise liquid or solid electrolytes.

When operating one exemplary embodiment of the DSSC 500 of the invention, the outside light is transmitted through the first electrode 200 into the Zn-doped TiO₂ porous layer 204 of the DSSC 500, and the Zn-doped TiO₂ porous layer 204 converts light energy from the light into electrical energy.

In one embodiment, a method for fabricating the DSSC 500 may comprise disposing the Zn-doped TiO₂ porous layer 204 between the first electrode 200 and the opposing second electrode 202. Next, an electrolyte 206 is disposed between the photo electrode 201 and the second electrode 202 using an injection or coating process. A method for fabricating the Zn-doped TiO₂ porous layer 204 may comprise steps as below.

Step (a): A TiO₂ powder, a Zn-containing compound and an alkaline aqueous solution are disposed in an autoclave and mixed to form a mixture. In one embodiment, the TiO₂ powder may comprise a commercial TiO₂ photocatalytic (Degussa P25). The Zn-containing compound may comprise Zn-containing inorganic metal salts or organic Zn-containing compounds, for example, Zn(NO₃)₂·6H₂O or Zn(CH₃COO)₂·xH₂O. The alkaline aqueous solution may comprise a sodium hydroxidean aqueous solution (NaOH), wherein the alkaline aqueous solution has a normality of between about 5 to 15 N, preferably about 10 N.

Step (b): Performing a first thermal process on the mixture. In one embodiment, the first thermal process may comprise a hydrothermal synthesis process, wherein the first thermal process may have a process time of between about 20 and 40 hours, preferably about 20 hours. The first thermal process may have a process temperature of between about 110° C. and 140° C., preferably about 130° C. After performing the first thermal process, a flaky mixture is formed.

Step (c): A washing process is performed on the mixture using an acidic aqueous solution to neutralize the alkaline aqueous solution until the pH value of the mixture reaches about 1.5 to 4. During this time, a Zn-doped TiO₂ multiple layer nano tube is formed. In one embodiment, the acidic aqueous solution may comprise an aqueous solution of nitric acid (HNO₃) or hydrogen chloride (HCl), wherein the acidic aqueous solution has a normality of between about 0.02 and 0.2N, preferably about 0.1N.

Step (d): A second thermal process is performed on the mixture to recrystallize the mixture, thereby more Zn atoms are doped in the TiO₂. After performing the second thermal process, Zn-doped TiO₂ powder is formed. In one embodiment, the second thermal process may comprise a hydrothermal synthesis process, wherein the first thermal process may have a process time of between about 10 and 20 hours, preferably about 12 hours. The second thermal process may have a process temperature, which is higher than the first thermal process, of between about 180° C. and 250° C., preferably about 240° C.

Step (e): A binder solution is mixed with the Zn-doped TiO₂ powder to form a paste. In one embodiment, a solvent of the binder solution may comprise H₂O, an alcohol of C₁₋₄, terpineol or carbitol. The binder of the binder solution may comprise sodium carboxymethyl cellulose, carboxymethyl cellulose (CMC), polyethylene glycol (PEG), ethyl cellulose (EC) or arcylic resin. In an embodiment of the binder solution formed by the H₂O solvent and the PEG binder, the Zn-doped TiO₂ powder may have a weight percentage of between about 30 wt % and 50 wt %, preferably about 40 wt %.

Step (f): The paste is coated on a first electrode 200 using a blade coating or screen printing process and then the paste is performed a sintering process to form a Zn-doped TiO₂ porous layer 204, wherein the Zn-doped TiO₂ porous layer 204 and the first electrode 200 construct a photo electrode 201.

Step (g): A second electrode 202 is disposed opposite to the photo electrode 201, so that the Zn-doped TiO₂ porous layer 204 is disposed between the first and second electrodes 200 and 202 with absorbing a dye.

FIG. 2 is an x-ray diffraction diagram showing the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of a photo electrode formed by a Zn-doped TiO₂ porous layer of the invention. Compared with a peak position of the conventional photo electrode formed by undoped TiO₂ photo-anode x-ray diffraction curve 20, peak positions of the exemplary embodiments including: a x-ray diffraction curve 22 of a photo electrode formed by a 0.2 at % (atom percent) Zn-doped TiO₂ porous layer (described as TZ02 hereinafter); a x-ray diffraction curve 24 of a photo electrode formed by a 0.4 at % Zn-doped TiO₂ porous layer (described as TZ04 hereinafter); a x-ray diffraction curve 26 of a photo electrode formed by a 0.7 at % Zn-doped TiO₂ porous layer (described as TZ07 hereinafter); and x-ray diffraction curve 28 of a photo electrode formed by a 1.2 at % Zn-doped TiO₂ porous layer (described as TZ12 hereinafter), all move toward a low diffraction angle. It is to be understood that in one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer, a Zn atom having a large diameter (Zn²⁺:0.74 Å, Ti⁴⁺:0.6 Å) may be introduced into the TiO₂ crystal lattice, thereby expanding the TiO₂ crystal lattice. From FIG. 2, one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer is represented by Ti_((1-x))Zn_(x)O₂.

Additionally, Zn-doped TiO₂ may be used in photocatalytic materials. In the fabrication process, Zn ion can be oxidized to form ZnO or be introduced into the TiO₂ crystal lattice, thereby changing the material composition of TiO₂. Therefore, Zn-doped TiO₂ photocatalytic performance under visible light may be changed. Zn-doped TiO₂ used in photocatalytic materials, however, has a purpose that is different from one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer of the invention. The Zn-doped TiO₂ photocatalytic materials may be used to provide oxide traps, thereby increasing absorption of light to improve photocatalytic activity. One exemplary embodiment of a Zn-doped TiO₂ photo-anode is used to provide an electron transport path with rapid transport velocity. Meanwhile, the fabricating method of one exemplary embodiment of a Zn-doped TiO₂ photo-anode is different from the Zn-doped TiO₂ photocatalytic.

FIG. 3 is a Mott-Schottky plot showing one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer, wherein Ag/AgCl is used as a reference electrode. FIGS. 4 a to 4 b are energy band diagrams showing the conventional photo electrode formed by undoped TiO₂ and one exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer. Table. 1 shows flat band voltage calculated from the intercept of FIG. 3 and donor density results calculated from FIG. 3.

TiO₂ TZ02 TZ04 TZ07 TZ12 flat band voltage (E_(F) −0.556 −0.568 −0.583 −0.640 −0.677 (V vs Ag/AgCl)) donor density (donor 3.4 × 3.9 × 4.8 × 4.6 × 4.5 × density (cm⁻³)) 10¹⁹ 10¹⁹ 10¹⁹ 10¹⁹ 10¹⁹

From FIG. 3 and Table 1, compared with the conventional photo electrode formed by undoped TiO₂ photo-anode (labeled as curve 30), one exemplary embodiment of the a photo electrode formed by TZ02 (labeled as curve 32), a photo electrode formed by the TZ04 (labeled as curve 34), a photo electrode formed by the TZ07 and a photo electrode formed by the TZ12 have higher flat band voltage and donor density. As shown in FIG. 4 a, the conventional photo electrode formed by undoped TiO₂ has a large number of unoccupied state traps. When the electrons are transported in the conventional photo electrode formed by undoped TiO₂, the electrons are easily captured by the unoccupied state traps, thereby generating electron loss. The electron transport path P₁ of the conventional photo electrode formed by undoped TiO₂ is not continuous. FIG. 4 b shows that one exemplary embodiment of a photo electrode formed by the Zn-doped TiO₂ porous layer has high Fermi energy E_(Fn), thereby causing a more curved energy band CB curve. The number of unoccupied state traps is thus reduced when compared to the conventional photo electrode formed by undoped TiO₂. Therefore, one exemplary embodiment of a photo electrode formed by the Zn-doped TiO₂ porous layer can provide a more continuous electron transport path P₂. Therefore, electron transportation efficiency is increased, so that the dye-sensitized solar cell still has high photoelectric conversion efficiency; especially in low light intensity environments.

FIG. 5 is a current density-voltage (J-V) curve comparison between the conventional photo electrode formed by undoped TiO₂ (thickness: 15 μm) and various exemplary embodiments of a photo electrode formed by a Zn-doped TiO₂ porous layer (thickness: 15 μm) of the invention. The current density versus voltage result is measured under a simulated sun light source (100 mW*cm⁻²) with a bias voltage. From FIG. 5, it is shown that the current density results of the photo electrode formed by the TZ04 (labeled as curve 54), the photo electrode formed by the TZ07 (labeled as curve 56) and the photo electrode formed by the TZ12 (labeled as curve 58) are of between about 14 mA*cm⁻² and 16 mA*cm⁻². The photo electrode formed by the TZ04 especially has the best current density result (16.2 mA*cm⁻²), while the photo electrode formed by the TZ07 has the next best current density result, which is almost the same with the conventional photo electrode formed by undoped TiO₂ (15.5 mA*cm⁻²).

FIG. 6 a is a short-circuit current density/light intensity-light intensity curve comparison between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of a photo electrode formed by a Zn-doped TiO₂ porous layer. FIG. 6 b is a photoelectric conversion efficiency-light intensity curve comparison between the conventional photo electrode formed by undoped TiO₂ and various exemplary embodiments of a photo electrode formed by a Zn-doped TiO₂ porous layer. From FIG. 6 a, it is shown that the dye-sensitized solar cell with the conventional photo electrode formed by undoped TiO₂ (labeled as curve 60) has a much reduced ratio to the short-circuit current density and light intensity (Jsc/I(A*W⁻¹)) when light intensity is reduced. The short-circuit current density/light intensity result of the dye-sensitized solar cell with the photo electrode formed by the TZ04 (labeled as curve 64) is not effected when light intensity is changed. The dye-sensitized solar cell with the photo electrode formed by the TZ04 has a high output current especially in low light intensity environments. The dye-sensitized solar cell with the photo electrode formed by the TZ07 (labeled as curve 66) has similar short-circuit current density/light intensity result to the dye-sensitized solar cell with the conventional photo electrode formed by undoped TiO₂ (labeled as curve 60). Further, the dye-sensitized solar cell with the photo electrode formed by the TZ07 (labeled as curve 66) has a better short-circuit current density/light intensity result than the conventional photo electrode formed by undoped TiO₂ (labeled as curve 60) in 60 to 80 mW*cm⁻² light intensity environments. If the doping content of the Zn-doped TiO₂ is too much, for example, such as the photo electrode formed by the TZ12 photo-anode (labeled as curve 68), the dye-sensitized solar cell with the Zn-doped TiO₂ photo-anode has a poorer short-circuit current density/light intensity result than that of the conventional photo electrode formed by undoped TiO₂ (labeled as curve 60). Additionally, as shown in FIG. 6 b, the dye-sensitized solar cell with the photo electrode formed by the TZ04 photo-anode (labeled as curve 74) has a photoelectric conversion efficiency that is 23% higher than the conventional photo electrode formed by undoped TiO₂ (labeled as curve 70). The dye-sensitized solar cell with the photo electrode formed by the TZ07 (labeled as curve 76) has similar photoelectric conversion efficiency results as the dye-sensitized solar cell with the conventional photo electrode formed by undoped TiO₂ (labeled as curve 70). Further, the dye-sensitized solar cell with the photo electrode formed by the TZ07 (labeled as curve 76) has a better photoelectric conversion efficiency result than the conventional photo electrode formed by undoped TiO₂ (labeled as curve 70) in 60 to 80 mW*cm⁻² light intensity environments. If the doping content of the Zn-doped TiO₂ porous layer is too much, for example, such as the photo electrode formed by the TZ12 (labeled as curve 78), crystal lattice defects occurs, thereby hindering electron transport in the photo-anode and reducing photoelectric conversion efficiency. Note that the photo electrode formed by the TZ12 is an example used for comparison.

One exemplary embodiment provides a dye-sensitized solar cell and a method for fabricating the same, wherein a photo electrode of the dye-sensitized solar cell is formed by a Zn-doped TiO₂ porous layer represented by Ti_((1-x))Zn_(x)O₂, wherein 0<x≦0.007. One exemplary embodiment of a photo electrode formed by a Zn-doped TiO₂ porous layer has high Fermi energy E_(Fn), thereby causing a more curved energy band CB curve. The number of unoccupied state traps of the photo electrode formed by the Zn-doped TiO₂ porous layer is reduced when compared to conventional photo electrode formed by undoped TiO₂. Therefore, one exemplary embodiment of the photo electrode formed by the Zn-doped TiO₂ porous layer can provide a more continuous electron transport path. Therefore, electron transportation may be increased, so that the dye-sensitized solar cell still has high photoelectric conversion efficiency and a high output current; especially in low light intensity environments.

While the invention has been described by way of example and in terms of the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements. 

1. A dye-sensitized solar cell, comprising: a photo electrode comprising a first electrode and a Zn-doped TiO₂ porous layer disposed on the first electrode, wherein the Zn-doped TiO₂ porous layer absorbs a dye; a second electrode disposed opposite to the photo electrode, wherein the Zn-doped TiO₂ porous layer is disposed between the first and second electrodes; and an electrolyte disposed between the photo electrode and the second electrode.
 2. The dye-sensitized solar cell as claimed in claim 1, wherein the Zn-doped TiO₂ porous layer is represented by Ti_((1-x))Zn_(x)O₂, wherein 0<x≦0.007.
 3. The dye-sensitized solar cell as claimed in claim 1, wherein the first electrode is a transparent electrode.
 4. The dye-sensitized solar cell as claimed in claim 1, wherein the second electrode comprises Pt, conductive polymer, carbon or composite materials.
 5. A method for fabricating a dye-sensitized solar cell, comprising: (a) mixing a TiO₂ powder, a Zn-containing compound and an alkaline aqueous solution to form a mixture and performing a thermal process on the mixture to form a Zn-doped TiO₂ powder; (b) mixing a binder solution with the Zn-doped TiO₂ powder to form a paste; (c) coating the paste on a first electrode, and the paste is performed a sintering process to form a Zn-doped TiO₂ porous layer, wherein the Zn-doped TiO₂ porous layer and the first electrode construct a photo electrode; (d) disposing a second electrode opposite to the photo electrode after a dye is absorbed by the Zn-doped TiO₂ porous layer; and (e) disposing an electrolyte between the photo electrode and the second electrode.
 6. The method for fabricating a dye-sensitized solar cell as claimed in claim 5, wherein the thermal process further comprising: (a) performing a first thermal process on the mixture; (b) performing a washing process on the mixture using an acidic aqueous solution; and (c) performing a second thermal process on the mixture.
 7. The method for fabricating a dye-sensitized solar cell as claimed in claim 5, wherein the Zn-containing compound comprises Zn-containing inorganic metal salts or organic Zn-containing compounds.
 8. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the first thermal process and the second thermal process comprise a hydrothermal synthesis process.
 9. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the first thermal process has a process time of between about 20 to 40 hours.
 10. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the first thermal process has a process temperature of between about 110° C. and 140° C.
 11. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the second thermal process has a process time of between about 10 and 20 hours.
 12. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the second thermal process has a process temperature of between about 180° C. and 250° C.
 13. The method for fabricating a dye-sensitized solar cell as claimed in claim 5, wherein the alkaline aqueous solution is a sodium hydroxide aqueous solution.
 14. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the acidic aqueous solution is a nitric acid or hydrogen chloride aqueous solution.
 15. The method for fabricating a dye-sensitized solar cell as claimed in claim 6, wherein the mixture after performing the washing process reaches a pH value of between about 1.5 and
 4. 16. The method for fabricating a dye-sensitized solar cell as claimed in claim 5, wherein the Zn-doped TiO₂ porous layer is represented by Ti_((1-x))Zn_(x)O₂, wherein 0<x≦0.007.
 17. The method for fabricating a dye-sensitized solar cell as claimed in claim 5, wherein a solvent of the binder solution comprises H₂O, an alcohol of C₁₋₄, terpineol or carbitol, and a binder of the binder solution comprises sodium carboxymethyl cellulose, carboxymethyl cellulose (CMC), polyethylene glycol (PEG), ethyl cellulose (EC) or arcylic resin. 